Existing oxidative dehydrogenation processes can be relatively impurity-sensitive and energy intensive due to the large recirculation rates of steam and nitrogen employed.
In conventional processes, air is used as the oxygen source for the reaction because, in part, nitrogen in the air acts as a diluent to moderate the intense exotherm of the reaction in order to control temperatures. Process details are discussed at some length in Welch et al., Butadiene via oxidative dehydrogenation, Hydrocarbon Processing, November 1978, pp. 131-136. The article notes that molecular oxygen, air, or mixtures of air and oxygen, can be used as the oxygen source, p. 131, and notes that certain impurities in the feed, such as isobutylene, reduces yields per pass (p. 136).
Sources of oxygen other than air for oxidative dehydrogenation to make butadiene are also discussed in the following references.
U.S. Pat. No. 3,327,001 to Tschopp discloses by implication that oxygen sources other than air may be used in an oxidative dehydrogenation butadiene process. See Col. 2, last paragraph which continues to Col. 3. U.S. Pat. No. 7,417,173, to Crone et al., has similar disclosure at Col. 7, lines 22-44. See, also, US 2008/0097133, of Crone et al., at paragraphs, [0050], [0051] and U.S. Pat. No. 7,435,860, to Crone et al., Col. 7, lines 12-35.
U.S. Pat. No. 8,088,962, to Klanner et al., discusses oxygen content of feed to oxidative dehydrogenation as including molecular oxygen and various diluents including optionally saturated hydrocarbons at Col. 18, lines 27-34. No examples of using oxygen or a saturated hydrocarbon diluent in an oxydehydrogenation unit are provided in this reference.
Although the use of pure oxygen in dehydrogenation processes for making butadiene has been suggested, in practice, air has been used as the oxygen source for cost reasons and because nitrogen present from the air-fed process operates to moderate temperature in processes for industrial production. Large scale processes require purification and recycle. Nitrogen present, a non-condensable gas in the process, contributes substantially to operating costs.
In a traditional version of the oxidative dehydrogenation process, an excess flow of steam to n-butene molar ratio of 12:1 is used to control the exotherm. The temperature of the reactor feed also needs to be increased to around 750° F. The air flow to the reactor is carefully controlled to promote butadiene selectivity, while ensuring that little or no oxygen passes beyond the reactors. Suitably, the air flow is measured and oxygen feed is calculated. Flow is then adjusted so that the amount of oxygen present falls within optimal range for reaction selectivity.
Nitrogen acts as diluent for the hydrocarbon-rich reactor contents. It absorbs part of the heat of reaction thus helping control the exotherm as the reaction proceeds. It also prevents the formation of a flammable hydrocarbon/oxygen mixture in the event of a process upset that “kills” the oxidation reaction.
Unlike steam, which can be removed from the reactor effluent early in the process via condensation in the quenching section, nitrogen is not condensable at normal process conditions. It remains part of the product stream as this stream moves through compression, scrubbing, and absorption sections, and it drives the equipment size, the piping size, as well as the overall design considerations. The nitrogen content increases significantly from 15 wt % (reactor outlet), to 45 wt % (gas compressor outlet), to 47 wt % (scrubber O/H), to 85 wt % (absorber O/H) in a typical air-fed process.
This high 85 wt % nitrogen content of the absorber O/H correlates with a low heating value for the stream (˜400 BTU/LB) and makes it unacceptable as boiler feed.
N-butene raw material for making butadiene is oftentimes scarce and difficult to obtain at prices suitable for commercial manufacturing operations. It is known in the art to dimerize ethylene to butene and use the recovered butene for manufacturing butadiene. U.S. Pat. No. 3,728,415 to Arganbright discloses producing butenes by dimerizing ethylene with a catalyst including palladium oxide with molybdenum oxide or tungsten oxide and using the product for dehydrogenation to make butadiene.
Other references of general interest include the following: U.S. Pat. Nos. 3,911,042 and 3,969,429 to Belov et al. which disclose titanium/aluminum catalyzed dimerization of ethylene and note the product is useful for making butadiene; U.S. Pat. No. 7,488,857 to Johann et al. which discloses coproduction of butadiene and butene-1 from butane; and United States Patent Application Publication No. US 2011/0288308 to Grasset et al. which discloses ethylene dimerization with titanium/aluminum catalyst.
It is proposed in Japanese Patent Publication 2011-148720 to manufacture butadiene from ethylene by way of dimerizing ethylene followed by oxidative dehydrogenation using specified catalysts to minimize impact of various impurities. The method proposed includes the following steps (I) and (II): a step (I) for producing n-butene essentially free of isobutene by dimerizing ethylene at a reaction temperature of 150 to 400° C. in the presence of a catalyst consisting of nickel, alumina, and silica having a nickel content of 0.0001 to 1 wt. %; and a step (II) for producing butadiene by performing an oxidative dehydrogenation reaction on the n-butene obtained in said step (I) with oxygen at a reaction temperature of 300 to 600° C. in the presence of a complex metal oxide comprising molybdenum and bismuth as essential ingredients.